Variable optical attenuator

ABSTRACT

A system and method is provided for attenuating a light signal. One embodiment includes a method for causing optical attenuation in a waveguide, where the waveguide has an input port for receiving a light signal and an output port for output of an attenuated light signal. First, an electric field is generated in at least a portion of the waveguide, such that a first refractive index in that portion of the waveguide is changed to a second refractive index. Next, the light signal in the waveguide is directed from the input port to the output port through the electric field. And lastly, the light signal is attenuated as a function of the electric field. The light signal may be attenuated, for example, by changing the deflection angle, changing the beam collimation width or from emitting part of the light signal from the waveguide before the light signal reaches the output port.

FIELD OF THE INVENTION

[0001] The invention relates generally to the field of opticalcommunications networks, and in particular to a variable opticalattenuator device.

BACKGROUND OF THE INVENTION

[0002] As bandwidth requirements of communication networks continue torise, wavelength division multiplexing is being used increasingly toaggregate the traffic of many users onto the optical fiber of backbonenetworks. For example, using a wavelength division multiplexer (WDM),Eighty or more separate wavelengths or channels of data can bemultiplexed into a light signal transmitted on a single optical fiber.If each channel carries 2.5 Gbps (billion bits per second), up to 200billion bits per second can be delivered on the single optical fiber.

[0003] However, in wavelength division multiplexing systems, the signalpower levels transmitted in an optical fiber depend on the wavelengths.These inter-wavelength discrepancies in optical power levels are causedin part by the use of optical amplifiers, such as erbium-doped fiberamplifiers (EDFAs). The use of EDFAs has revolutionized fiber optics, asthey enable WDM data transport over thousands of kilometers.Unfortunately, as EDFAs do not inherently have a flat gain spectrum,there is the problem of uneven gain for different wavelengths. VariableOptical Attenuators (VOAs) provide a solution to this problem byattenuating different wavelengths by different amounts, thereforeflattening the gain spectrum.

[0004]FIG. 1 is a schematic of an exemplary prior aft application of avariable optical attenuator (VOA). A multiple number of VOAs, e.g.,110-1, 110-2, and 110-3, each receive an input wavelength, e.g., λ1, λ2,and λn, respectively. The VOAs attenuate the power of each inputwavelength by different amounts and then transmit the attenuatedwavelengths to the WDM 112 to be multiplexed into a multi wavelengthlight signal. This multi wavelength light signal is the amplified by anEDFA optical amplifier 114 and output to a fiber optic cable fortransmission. The attenuation for each VOA has been chosen to compensatefor the uneven gain spectrum of the optical amplifier 114.

[0005] VOAs in current use include either Mach-Zender interferometerswhich use a thermo-optic effect to cause variation in attenuation or anelectronically controlled mechanical means to cause variation inattenuation. One of the significant disadvantages of these typical VOAsis the speed (i.e., long settling or slow response times). Hence for thefast optical switching networks, which need high speed power adjustmentson the order of about one nanosecond (1 GHz), current VOAs areinadequate. Therefore what is needed is a VOA with high speedattenuation adjustment that can support fast optical switching networks.

SUMMARY OF THE INVENTION

[0006] The present invention provides techniques, including a system andmethod, for attenuating a light signal using the electro-optic effect toprovide fast attenuation adjustment. One embodiment of the presentinvention comprises a method for causing optical attenuation in awaveguide, where the waveguide has an input port for receiving a lightsignal and an output port for output of an attenuated light signal.First, an electric field is generated in at least a portion of thewaveguide, such that a refractive index in that portion of the waveguideis changed. Next, the light signal in the waveguide is directed from theinput port to the output port through the electric field. And lastly,the light signal is attenuated as a function of the electric field. Thelight signal may be attenuated, for example, by changing the deflectionangle, changing the beam collimation width or from emitting part of thelight signal from the waveguide before the light signal reaches theoutput port.

[0007] Another embodiment of the present invention comprises a VOA forattenuating a light signal. The VOA includes: a waveguide, having aninput port for receiving the light signal and an output port for outputof an attenuated light signal; a first lens for collimating the receivedlight signal; a prism formed by an electric field, where the prismchanges a deflection of said collimated light signal depending on thestrength of the electric field; and a second lens for focusing thechanged light signal on or near the output port, where the attenuationof the light signal is dependent on a location of a focal point of thefocused light signal with respect to the output port. The location ofthe focal point varies as a function of the electric field.

[0008] A further embodiment of the present invention comprises a VOA forattenuating a light signal. The VOA includes: a waveguide, having aninput port for receiving the light signal and an output port for outputof an attenuated light signal; a first lens for collimating the receivedlight signal; a second lens formed by an electric field, where thesecond lens causes a change in a collimation width of the collimatedlight signal depending on a strength of the electric field; and a thirdlens for focusing the changed collimated light signal, where attenuationof the light signal is dependent on the changed collimation width.

[0009] An alternative embodiment of the present invention comprises aVOA for attenuating a light signal. The VOA includes: a waveguide,having an input port for receiving said light signal and an output portfor output of an attenuated light signal; a first electric field in saidwaveguide for collimating said received light signal; a second electricfield in said waveguide for changing said collimated light signaldepending on a strength of said second electric field; and a thirdelectric field in said waveguide for focusing said changed light signalat or near said output port, wherein attenuation of said light signal isdependent on a location of a focal point of said focused light signalwith respect to said output port.

[0010] Yet another embodiment of the present invention comprises a VOAfor attenuating a light signal. The VOA includes: an input port forreceiving the light signal; an output port; a waveguide for propagatingthe light signal from the input port to the output port, and a topelectrode on the top clad layer for creating an electric field, wherethe electric field changes a refractive index of a portion of the topclad layer, such that a part of the light signal is emitted out of thewaveguide before the output port. The waveguide includes a core, a topclad layer, and a bottom clad layer, where a part of the top clad layerhas an electro-optic material.

[0011] One aspect of the present invention comprises a system forattenuating a light signal. The system includes: a waveguide comprisinginput means for receiving the light signal and an output port; means forgenerating an electric field in at least a portion of the waveguide suchthat a first refractive index in the portion of the waveguide changesthe refractive index; means for directing the light signal in thewaveguide from the input means to the output port through the electricfield; and means for attenuating the light signal in the waveguide as afunction of the electric field.

[0012] These and other embodiments, features, aspects and advantages ofthe invention will become better understood with regard to the followingdescription, appended claims and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a schematic diagram of a know use of a variable opticalattenuator (VOA);

[0014]FIG. 2 is a cross-section of a portion of a waveguide of anembodiment of the present invention;

[0015]FIG. 3 is a top view of a VOA device of an embodiment of thepresent invention, which varies optical attenuation by changing theoptical beam deflection angle;

[0016]FIG. 4 is a top view of a VOA device of another embodiment of thepresent invention, which varies optical attenuation by changing theoptical beam collimation;

[0017]FIG. 5 is a cross-sectional view of a VOA device of yet anotherembodiment of the present invention, which attenuates the light signalby introducing mode mismatch inside the waveguide;

[0018]FIG. 6 is an isometric view of a waveguide having a S-shaped corechannel of another embodiment of the present invention;

[0019]FIG. 7 is an isometric view of a waveguide having a straight corechannel of an alternative embodiment of the present invention;

[0020]FIG. 8 is a cross-sectional view of the waveguide of FIG. 6;

[0021]FIG. 9 is a cross-sectional view of the waveguide of FIG. 7; and

[0022]FIG. 10 is a graph showing simulation results for the amount ofattenuation for a given applied voltage for the waveguides of FIGS. 6and 7.

DETAILED DESCRIPTION

[0023] In the following description, numerous specific details are setforth to provide a more thorough description of the specific embodimentsof the invention. It will be apparent, however, to one skilled in theart, that the invention may be practiced without all the specificdetails given below. In other instances, well known features have notbeen described in detail so as not to obscure the invention.

[0024]FIG. 2 is a cross-section of a portion of a slab waveguide of anembodiment of the present invention. The slab waveguide includes a core214 surrounded by cladding material. In FIG. 2 the surrounding claddingmaterial is depicted as top clad layer 212 and bottom clad layer 216. Ontop of and affixed to the top clad layer 212 is a top electrode 218.Below and affixed to the bottom clad layer 216 is a bottom electrode220. A first voltage applied to top electrode 218 and a second voltageapplied to bottom electrode 220 causes an electric field between theelectrodes. A light signal 222 propagates through the core 214. Whenthere is no electric field, the top clad layer 212 has refractive indexn1; the core has refractive index n2; and the bottom clad layer 216 hasrefractive index n3. The refractive index of the core 214 is greaterthan the surrounding cladding material, i.e., n2>n1 and n2>n3. The coreis made of an electro-optic (EO) material such as ferroelectric oxide,e.g., LiNbO3, PZT, or PLZT, that changes its refractive index in thepresence of an electric field. Hence by making one or both of theelectrodes a certain shape, an electrically formed lens (or prism) iscreated when voltages are applied to the top and bottom electrodes,e.g., 218 and 220. In addition, any refractive index change of the topor bottom clad layers (212 and 216) in the presence of the electricfield generated by voltages to electrodes 218 and 220 is less than therefractive index change in portion 214, allowing the slab to function asa waveguide in the presence of an electric field.

[0025]FIG. 3 is a top view of a variable optical attenuator (VOA) device308 of an embodiment of the present invention, which varies opticalattenuation by changing the optical beam deflection angle. In FIG. 3 thetop view shows the cores of the waveguides, i.e., core channels 312 and316 and the slab waveguide core 314. The lenses or prisms, e.g., 320,322, 324, and 326, are located in the slab waveguide core 314. In oneembodiment these lenses or prisms are formed from electric fieldsproduced by electrodes above and below the slab waveguide core 314. Inan alternative embodiment only prisms 322 and 324 are electricallyformed, while lenses 320 and 326 are physical lenses. Channel waveguide313 with core channel 312 is optically coupled to slab waveguide core314. Slab waveguide core 314 is optically coupled to channel waveguide315 with core channel 316. The VOA 308 has longitudinal axis 310illustrated by a horizontal dotted line. Slab waveguide core 314includes collimating lens 320, prism 322, prism 324, and focusing lens326. The number, type, and location of the lenses and prisms may vary inother embodiments of the present invention to achieve opticalattenuation.

[0026] A light signal 330 enters the VOA 308 at input port 331 andpropagates through channel waveguide 313, slab waveguide 314, andchannel waveguide 315 to output port 340. In channel waveguide 313 thelight signal 330 travels through channel 312, and at channel the exit332, the light signal 330 enters slab waveguide core 314. The diverginglight signal is collimated by collimating lens 320 into light beam 333.Light beam 333 is deflected, i.e., the light beam's direction ischanged, by prism 322 to give light beam 334. The amount of defection isdependent upon the strength of the electric field produced in thewaveguide 314 by the electrodes (not shown) of electrically formed prism322. The electrically formed prism 324 changes the direction of lightbeam 334 to be parallel to, but offset from longitudinal axis 310. Lightbeam 335 is then converged by focusing lens 326 to focal point 336,which located at or near the input 337 of core channel 316 of channelwaveguide 315. The focused light beam 341 then proceeds as an attenuatedlight signal 338 to output port 340 via core channel 316.

[0027] When the voltages are off, prisms 322 and 324 are not formed, andthe light beam 333 proceeds along the longitudinal axis 310 to lens 326,where the light beam 333 is focused to a focal point 336 located on thelongitudinal axis 310. The amount of attenuation should be at a minimumfor this case. By changing the amount of deflection of light signal 333produced by prism 322, the focal point 336 moves up and down the normalto longitudinal axis 310, i.e., it is offset. The further the focalpoint is located away from the longitudinal axis 310, the greater theattenuation as less light enters channel 316. Thus the amount of lightattenuation may be varied as a function of the electric field of prism322, i.e., by the amount of defection of the light beam.

[0028]FIG. 4 is a top view of a VOA 408 of another embodiment of thepresent invention, which varies optical attenuation by changing theoptical beam collimation. In FIG. 3 the top view shows the cores of thewaveguides, i.e., core channels 412 and 416 and the slab waveguide core414. The lenses, e.g., 420, 422, 424, and 426, are located in the slabwaveguide core 414. In one embodiment these lenses are formed fromelectric fields produced by electrodes (not shown) above and below theslab waveguide core 414. In an alternative embodiment only lenses 422and 424 are electrically formed, while lenses 420 and 426 are physicallenses. Channel waveguide 413 with core channel 412 is optically coupledto slab waveguide core 414. Channel waveguide 413, having channel 412,is optically coupled to slab waveguide core 414. Slab waveguide 414 isoptically coupled to channel waveguide 415, having core channel 416. TheVOA has longitudinal axis 410 illustrated by a horizontal dotted line.Slab waveguide core 414 includes collimating lens 420, diverging lens422, collimating lens 424, and focusing lens 426. The number, type, andlocation of the lenses and prisms may vary in other embodiments of thepresent invention to achieve optical attenuation.

[0029] A light signal 430 enters the VOA 408 at input port 431 andpropagates through channel waveguide core 412, slab waveguide core 414,and channel waveguide core 416 to output port 440. In channel waveguidecore 414 the light signal 430 travels through core channel 412, and atchannel exit 432 to channel 412, the light signal 430 diverges into slabwaveguide core 414. The diverging light signal is collimated bycollimating lens 420 into collimated light beam 433 with a width 450.Diverging lens 422 causes light beam 434 to spread out. The amount ofdivergence is dependent upon the strength of the electric field producedin the slab waveguide core 413 by the electrodes of diverging lens 422.The collimator lens 424 re-collimates light beam 434 to a light beam 435with a width 452 of the re-collimated beam that is greater than thewidth 450 of the collimated light beam 433. Light beam 435 is thenconverged by focusing lens 426 to focal point 436 which is located alonglongitudinal axis 410 at or near the entrance 437 to channel 416 ofchannel waveguide 415. The focused light beam 441 then proceeds asattenuated light signal 438 to output port 440 via core channel 416 ofchannel waveguide 415. In an alternative embodiment the re-collimatedbeam width 452 is less than the width 450 of the collimated light beam433.

[0030] When the voltages are off, lenses 422 and 424 are not formed, andthe light beam 433 proceeds along the longitudinal axis 410 to lens 426where the light beam 433 is focused to a focal point 436 located on thelongitudinal axis 410 at or near channel entrance 437. The amount ofattenuation is at a minimum for this case. By increasing the amount ofdivergence of light signal 433 produced by diverging lens 422, thecollimation width 452 is increased, and the amount of light from focusedlight beam 441 that goes through entrance 437 is decreased. In otherwords, the amount of light attenuation is a function of the width 452 ofthe collimation of the light beam. In an alternative embodiment thefocal point 436 may also be moved along longitudinal axis 410 bychanging the electric field of focusing lens 436, hence changing therefractive index of lens 426 with respect to the refractive index of theslab waveguide core 414.

[0031]FIG. 5 is a cross-sectional view of a VOA 508 of yet anotherembodiment of the present invention, which attenuates the light signalby introducing mode mismatch inside the waveguide. The waveguideincludes a top clad layer 510, a core 514, and a bottom clad layer 516.The top clad layer 510 includes a portion 512 having an electro-optic(EO) material, such as LiNbO3, PZT, or PLZT. Positioned on top of andaffixed to portion 512 of top clad layer 510 is a top electrode 520.Bottom electrode 522 is positioned below and affixed to bottom cladlayer 516. The top 520 and bottom 522 electrodes, when there is avoltage applied, produces an electric field 524 (illustrated by thedotted arrows) in the waveguide. The electric field in portion 512increases the refractive index of portion 512 to a higher refractiveindex value n4 (where n4>n1). The refractive indexes of the core 514 andbottom clad layer 516 remain the same or change less than that ofportion 512, whether or not there is an electric field.

[0032] Changing the refractive index of the top clad layer 510 inportion 512, causes some light to pass out (or “leak out”) of thewaveguide. For example, the mode field diameter of a step-indexed fiberis a function of the core diameter, wavelength, and the refractiveindexes of the core and clad. As the refractive indexes of the core andclad layers are brought closer together, for example by increasing therefractive index of portion 512, the mode field diameter gets larger,and the power propagating along core 514 decreases. Specifically, whenthe refractive index of the top clad layer 510 is increased by theelectric field in portion 512, the beam power confined in the core 514in the vicinity of portion 512 decreases. Some portion of the lightpasses from core 514 into portion 510, as represented by light ray 542,and the light propagating down core 514 is attenuated.

[0033] For example, light rays 530 and 532 are normally reflected at thecore-clad interface as they propagate along core 514. With no electricfiled, both rays will propagate to the other end of the core 514. Whenelectric field 524 increases the refractive index of portion 512, ray532 at the core-clad interface 540, is refracted out of the core 514rather than being reflected (the top electrode 520, in this case, istransparent). The electric field effectively decreases the criticalangle needed for total reflection, so light ray 532 is no longerreflected at interface 540. Light ray 530 continues to be totallyinternally reflected.

[0034]FIG. 6 shows a waveguide 610 having an S-shaped core channel 618of another embodiment of the present invention. Waveguide 610 has topclad layer 612 having an EO material and a bottom clad layer 616 with anon-EO material. The core channel 618 has a non-EO material and isformed within the bottom clad layer 616 (see FIG. 9 for cross-sectionalview). The core 618 has a curved shape, e.g., S-shape. In otherembodiments the core 618 maybe straight or otherwise curved. The core618 dimensions include the width 630 of the bottom clad layer 616 andthe lateral shift 634 from the entrance 636 into core 618 to the exit638 from core 618. A light signal 620 enters the core channel 618 atentrance 636 and propagates in the core channel 618 as light signal 622until the channel exit 638. Electrodes (not shown) are positioned abovetop clad layer 612 and below bottom clad layer 616 to create an electricfield in and around core channel 618. The electric field increases theindex of refraction in the top clad layer and causes' a portion of thelight signal 622 to leak out of the waveguide 610. In addition there issignificant light leakage in the curved areas 640 and 642 of core 618,because the critical angles needed for total reflection of the beam canno longer be met due to the bends in the core. Thus light signal 624 isattenuated by controlling the electric field, i.e., the voltage on theelectrodes.

[0035]FIG. 7 shows a waveguide 650 having a straight core channel 656 ofanother embodiment of the present invention. Waveguide 650 has top cladlayer 652 having an EO material and a bottom clad layer 654 with anon-EO material. The core channel 656 has a non-EO material and isformed within the top clad layer 652 (see FIG. 8 for cross-sectionalview). The core 656 has a non-curved shape, e.g., straight. In otherembodiments, the core 656 has a curved shape, e.g., S-shape. A lightsignal 660 enters the core channel 656 at entrance 664 and propagates inthe core channel 656 until the channel exit 666. Electrodes (not shown)are positioned above top clad layer 652 and below bottom clad layer 654to create an electric field in and around core channel 656. The electricfield increases the index of refraction in the top clad layer 652 andcauses a portion of the light signal in core 656 to leak out of thewaveguide 650. Thus light signal 662 is an attenuated version of lightsignal 660, where the attenuation is controlled by controlling theelectric field, i.e., the voltage on the electrodes.

[0036]FIG. 8 is a cross-sectional view of the waveguide 650 of FIG. 7along view line YY. The waveguide 710 includes top clad layer 712, core714, and bottom clad layer 716. Top clad layer 712 comprises EOmaterial. Core 714 is formed within top clad layer 712. The core 714 andbottom clad layer 716 have non-EO material. FIG. 8 shows a ridge typeplacement of the core 714 above the bottom clad layer 716.

[0037]FIG. 9 is a cross-sectional view of the waveguide 610 of FIG. 6along view line XX. The waveguide 810 includes top clad layer 812, core814, and bottom clad layer 816. Top clad layer 812 has EO material. Thecore 814 is formed within bottom clad layer 816. The core 814 and bottomclad layer 816 have non-EO material. FIG. 8 shows a buried typeplacement of the core 814 in the bottom clad layer 816.

[0038] A simulation was conducted using the waveguide 650 having thestraight channel core with both ridge type (FIG. 8) and buried type(FIG. 9) cores. The simulation also used using the waveguide 610 havingthe S-channel core with both ridge type (FIG. 8) and buried type (FIG.9) cores. The refractive index of the bottom clad was about 1.563. Therefractive index of the core was about 1.567. The refractive index ofthe top clad (with the EO material having electro-optic coefficient of100 picometers per volt) was about 1.563. For FIG. 6 the width 630 (and632) was about 4 mm and the lateral shift 634 about 0.125 mm. The corehad a 7×7 μm cross-section.

[0039]FIG. 10 is a graph showing the results of a simulation for theamount of attenuation for a given applied voltage for the waveguides ofFIGS. 6 and 7. The y-axis 912 gives the attenuation in dB (power) per cmand the x-axis 910 gives voltage applied across the electrodes in voltsper 10 μm. The EO coefficient is 100 ρm/volt. Curves 920 and 922 showthe attenuation for a curved core such as in FIG. 6, and curves 924 and926 show attenuation for a straight core such as in FIG. 7. Curves 920and 924 are for the ridge type core of FIG. 8. Curves 922 and 926 arefor the buried type core of FIG. 9. From curves 920 and 922, theS-shaped core channel of FIG. 6 gives a wider dynamic range, .e.g., >15dB, when compared to the straight channel core of FIG. 7. However, thestraight core channel (curves 924 and 926) does allow finer control ofthe attenuation.

[0040] The specification and drawings are to be regarded in anillustrative rather than a restrictive sense. It will, however, beevident that additions, subtractions, deletions, and other modificationsand changes may be made thereunto without departing from the broaderspirit and scope of the invention as set forth in the claims.

What is claimed is:
 1. A method for causing optical attenuation in awaveguide, said waveguide comprising an input port for receiving a lightsignal and an output port for output of an attenuated light signal, saidmethod comprising: generating an electric field in at least a portion ofsaid waveguide such that a first refractive index in said portion ofsaid waveguide is changed to a second refractive index; directing saidlight signal in said waveguide from said input port to said output portthrough said electric field; and attenuating said light signal as afunction of said electric field.
 2. The method of claim 1, wherein saidwaveguide further comprises a core and a clad, said clad comprising saidportion, and wherein increasing said electric field increases lightemission loss from said waveguide.
 3. The method of claim 1 wherein saidwaveguide further comprises a focusing lens near said output port andwherein said attenuating said light signal comprises changing a positionof a focal point of said focusing lens with respect to said output port,said changing of said focal point based on said function of saidelectric field.
 4. The method of claim 1 wherein said generating anelectric field comprises forming a diverging lens.
 5. The method ofclaim 4 further comprising: collimating said light signal from saidinput port; changing a width of collimation of said collimated lightsignal using said diverging lens, wherein said width is dependent on amagnitude of said electric field; and focusing said collimated lightsignal having said changed width.
 6. The method of claim 1 wherein saidgenerating an electric field comprises forming a prism for deflectingsaid light signal.
 7. The method of claim 6 further comprising:collimating said light signal from said input port; changing beamdirection of said light signal using said prism, wherein said changingis dependent on a magnitude of said electric field; and focusing saidlight signal.
 8. A variable optical attenuator for attenuating a lightsignal comprising: a waveguide, comprising an input port for receivingsaid light signal and an output port for output of an attenuated lightsignal; a first lens for collimating said received light signal; a prismformed by an electric field, said prism changing a deflection of saidcollimated light signal depending on a strength of said electric field;and a second lens for focusing said changed light signal, whereinattenuation of said light signal is dependent on a location of a focalpoint of said focused light signal with respect to said output port,said location varying as a function of said electric field.
 9. Thevariable optical attenuator of claim 8 wherein said waveguide furthercomprises a core having a straight shape.
 10. The variable opticalattenuator of claim 8 wherein said waveguide further comprises a corehaving a curved shape.
 11. A variable optical attenuator for attenuatinga light signal comprising: a waveguide, comprising an input port forreceiving said light signal and an output port for output of anattenuated light signal; a first lens for collimating said receivedlight signal; a second lens formed by an electric field, said secondlens causing a change in a collimation width of said collimated lightsignal depending on a strength of said electric field; and a third lensfor focusing said changed collimated light signal, wherein attenuationof said light signal is dependent on said changed collimation width. 12.The variable optical attenuator of claim 11 wherein said second lenscauses said collimated light signal to diverge and further comprising afourth lens for re-collimating said diverged light signal.
 13. Thevariable optical attenuator of claim 11 wherein said third lens isformed from a second electric field, and further comprising: a focalpoint of said focused changed collimated light signal, whereinattenuation of said light signal is further dependent on a location ofsaid focal point, said location varying as a function of said secondelectric field.
 14. The variable optical attenuator of claim 11 whereinsaid waveguide further comprises a core having a curved shape.
 15. Thevariable optical attenuator of claim 11 wherein said waveguide furthercomprises a core having a straight shape.
 16. A variable opticalattenuator for attenuating a light signal comprising: a waveguide,comprising an input port for receiving said light signal and an outputport for output of an attenuated light signal; a first electric field insaid waveguide for collimating said received light signal; a secondelectric field in said waveguide for changing said collimated lightsignal depending on a strength of said second electric field; and athird electric field in said waveguide for focusing said changed lightsignal at or near said output port, wherein attenuation of said lightsignal is dependent on a location of a focal point of said focused lightsignal with respect to said output port.
 17. A variable opticalattenuator for attenuating a light signal comprising: an input port forreceiving said light signal; an output port for output of saidattenuated light signal; a waveguide for propagating said light signalfrom said input port to said output port, said waveguide comprising acore, a top clad layer, and a bottom clad layer, wherein a part of saidtop clad layer comprises an electro-optic material; and a top electrodepositioned on said top clad layer for creating an electric field, saidelectric field changing a refractive index of a portion of said top cladlayer such that a part of said light signal is emitted out of saidwaveguide before said output port.
 18. The variable optical attenuatorof claim 17 wherein said bottom clad layer comprises a material whoserefractive index is unchanged when said electric field is present. 19.The method of claim 17 further comprising a bottom electrode affixed tosaid bottom clad layer.
 20. The method of claim 17 wherein when there isno electric field said core has a higher refractive index than said topclad layer.
 21. The method of claim 17 wherein said core is affixed onsaid bottom clad layer and is formed within said top clad layer.
 22. Thevariable optical attenuator of claim 21 wherein said core curves fromsaid input port to said output port.
 23. The variable optical attenuatorof claim 21 wherein said bottom clad layer comprises a material whoserefractive index is unchanged when said electric field is present. 24.The variable optical attenuator of claim 21 wherein from a top-view ofsaid waveguide said core has a shape selected from a group consisting ofa curve and a straight line.
 25. The variable optical attenuator ofclaim 17 wherein said core is affixed below said top clad layer and isformed within said bottom clad layer.
 26. The variable opticalattenuator of claim 25 wherein said bottom clad layer comprises amaterial whose refractive index is unchanged when said electric field ispresent.
 27. The variable optical attenuator of claim 25 wherein saidcore curves from said input port to said output port.
 28. The variableoptical attenuator of claim 25 wherein from a top-view of said waveguidesaid core has a shape selected from a group consisting of a straightline and a curve.
 29. A system for attenuating a light signalcomprising: a waveguide comprising input means for receiving said lightsignal and an output port; means for generating an electric field in atleast a portion of said waveguide such that a first refractive index insaid portion of said waveguide is changed to a second refractive index;means for directing said light signal in said waveguide from said inputmeans to said output port through said electric field; and means forattenuating said light signal in said waveguide as a function of saidelectric field.